In the past, an electrolyte in which a film having micro-pores called a separator was impregnated with a non-aqueous electrolytic solution was generally used. A lithium ion secondary battery called a polymer battery employing a polymer electrolyte made of a polymer has recently attracted more attention than such electrolyte employing liquid.
This polymer battery uses an electrolyte made in the form of gel in which the polymer is impregnated with a liquid electrolytic solution. Since it holds a liquid electrolytic solution in the polymer, it has the advantages that there is little possibility of leakage of the liquid and, therefore, safety of the battery is improved and that it has more freedom in adopting the configuration of the battery.
Since lithium ion conductivity of such polymer electrolyte is lower than an electrolyte containing only an electrolytic solution, there has occurred a practice to reduce thickness of the polymer electrolyte. There, however, has arisen a problem in such polymer electrolyte whose thickness is reduced that, since its mechanical strength is reduced, the polymer electrolyte tends to be broken or give rise to a hole during production of the battery resulting in short-circuiting between the positive electrode and the negative electrode.
It has, therefore, been proposed, as disclosed by Japanese Patent Application Laid-open Publication No. 6-140052, to provide a solid electrolyte by adding an inorganic oxide such as alumina to the electrolyte and thereby increase its mechanical strength. As such inorganic oxide, inorganic oxides other than alumina such as silica and lithium aluminate have also been proposed.
However, the addition of such inorganic oxides such as alumina to an electrolyte causes the problem that lithium ion conductivity in the solid electrolyte is significantly reduced. Moreover, when charging and discharging are repeated in a lithium ion secondary battery employing this solid electrolyte, the electrolyte reacts with such inorganic oxide resulting in deterioration in the charging-discharging characteristic of the lithium ion secondary battery.
An all solid battery employing an inorganic solid electrolyte as an electrolyte of a lithium ion secondary battery has also been proposed. The all solid battery is superior in its safety because it does not use a combustive organic solvent such as an electrolytic solution and therefore there is no danger of leakage of liquid or combustion. In the all solid battery, however, all of its positive electrode, electrolyte and negative electrode are made of solid and, therefore, close contacts between each of these components is hard to realize and, as a result, interface resistance tends to increase. In this case, since resistance to movement of lithium ion at the interfaces between the electrodes and the electrolyte is so large that it is difficult to achieve a battery having a high output.
Further, as disclosed for example by Japanese Patent Application Laid-open Publication No. 2004-348972, there it been reported that a lithium ion secondary battery can be assembled by using an all solid electrolyte made by pelletizing solid inorganic substance such as sulfide glass by pressing. Since, however, this secondary battery has not sufficient ion conductivity to enable it to be used for practical purposes.
It is, therefore, an object of the present invention to solve the problem of difficulty in putting a solid electrolyte and a lithium ion secondary battery or a lithium primary battery using the solid electrolyte to practical uses due to low lithium ion conductivity.
It is another object of the present invention to provide a solid electrolyte and a lithium primary battery and a lithium ion secondary battery using a solid electrolyte which, notwithstanding that it does not use an electrolytic solution, has a high battery capacity and an excellent charging and discharging characteristic, can be used stably over a long period of time and is easy to manufacture and treat in production in an industrial scale.